The instant invention is directed in general to combustors for gas turbines and, more specifically, to a hybrid can-annular combustor for axial staging in low NOx Combustion.
Over the past ten years there has been a dramatic increase in the regulatory requirements for low emissions from gas turbine power plants. Environmental agencies throughout the world are now requiring even lower rates of emissions of NOx and other pollutants from both new and existing gas turbines.
Traditional gas turbine combustors use nonpremixed diffusion flames in which fuel and air freely enter the combustion chamber separately. Typical diffusion flames are dominated by regions which burn at or near stoichiometric conditions. The resulting flame temperatures can exceed 3000.degree. F. Because diatomic nitrogen rapidly disassociates at temperatures exceeding about 3000.degree. F., diffusion flames typically produce unacceptably high levels of NOx emissions.
One method commonly used to reduce peak temperatures, and thereby reduce NOx emissions, is to inject water or steam into the combustor. However, water/steam injection is a relatively expensive technique and can cause the undesirable side effect of quenching carbon monoxide (CO) burnout reactions. Additionally, water/steam injection methods are limited in their ability to reach the extremely low levels of pollutants required in many localities. More frequent combustion inspections and decreased hardware life are additional side effects that can result from the use of water/steam injection methods to reduce NOx emissions from combustion turbines.
Lean premixed combustion is a much more attractive method of lowering peak flame temperatures, and correspondingly, NOx emission levels. In lean premixed combustion, fuel and air are premixed in a pre-mixing section, and the fuel-air mixture is injected into a combustion chamber where it is burned. Due to the lean stoichiometry resulting from the premixing, lower flame temperatures and NOx emission levels are achieved.
Several types of low NOx emission combustors are currently employing lean-premixed combustion for gas turbines, including can and annular type combustors.
Can combustors typically consist of a cylindrical can-type liner inserted into a transition piece with multiple fuel-air premixers positioned at the head end of the liner. Such an arrangement is illustrated in FIG. 1. Although this system is practical and easy to assemble, it has several inherent disadvantages for achieving ultra-low emissions and maximum operability.
Can combustors are relatively lengthy and provide a long combustor residence time. During low load and/or low temperature operation, the levels of carbon monoxide (CO) and unburned hydrocarbon are minimized due to the long combustor residence time.
However, during high load and/or high temperature operation, diatomic nitrogen begins to rapidly disassociate, and NOx emissions grow in time. Therefore, the large residence time of the can combustor results in high NOx emissions during high-load and/or high temperature operation.
Annular combustors are also used in many gas turbine applications. Annular combustors typically consist of multiple premixers positioned in rings directly upstream of the turbine nozzles in an annular fashion. Such an arrangement is illustrated in FIG. 2. The annular combustor is short in length and accordingly, has a relatively short combustor residence time.
During high load and/or high temperature operation, the levels of NOx emissions are low due to the short combustor residence time in the short annular combustor.
However, during low load and/or low temperature operation, the levels of carbon monoxide (CO) and unburned hydrocarbon (UHC) are large due to the short combustor residence time of the annular combustor, not allowing complete CO and UHC burnout.
Additionally, combustion instability poses serious limitations upon the operability of premixed combustion systems. The strength of such instabilities can be substantially reduced by spreading the combustion out within the combustor such that the process is not concentrated in one location. Accordingly, significant improvements in operability can be accomplished if the combustion is distributed along the axis of the combustor. This distribution is termed axial staging.
Therefore, it is apparent from the above that there exists a need in the art for a combustor which uses premixing stages at different axial positions to carry out axial staging of the heat release for increased operability, where the combustor has a first premixing means for injecting a fuel-air mixture into a can-type configuration for providing the advantages of a can type combustor for low load and/or low temperature conditions and a second premixing means for injecting a fuel air mixture into an annular type configuration for providing the advantages of an annular type combustor for high load and/or high temperature conditions. It is a purpose of this invention, to fulfill these and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.